Any discussion of the background art throughout the specification should in no way be considered as an admission that such background art is prior art, nor that such background art is widely known or forms part of the common general knowledge in the field.
The detection of substances posing chemical, biological and explosives (CBE) threats has become critical in recent years, for example, for airport security.
U.S. Pat. No. 6,795,190 (U.S. Ser. No. 09/976,549) issued 21 Sep. 2004 in the name of Paul et al describes an absorption spectroscopy instrument with off-axis light insertion into a cavity. This instrument is described as allowing optical cavities to be used simply and effectively as absorption cells for the purpose of performing sensitive absorption spectroscopy. A continuous-wave light beam is introduced into the cavity using an off-axis cavity alignment geometry, which eliminates resonances commonly associated with optical cavities, while preserving the absorption signal amplifying properties of such cavities. Doing so is said to reduce the complexity of the instrument compared with other optical cavity-based absorption methods when applied in conjunction with integrated cavity output spectroscopy (ICOS).
Such off-axis cavity-enhanced spectroscopy is a variant of cavity ringdown spectroscopy (CRDS) that uses off axis paths (that is, the laser is coupled into the linear cavity at an angle away from the major axis) through a two-mirror resonator to measure the absorbance from a sample within the cavity. The result of such an off-axis path is that the cavity's mode structure collapses, and light is able to couple with a cavity mode more frequently. The absorbance is commonly measured with off-axis integrated cavity output spectroscopy (OA-ICOS): a detector is placed behind one of the cavity mirrors, and the intensity of the light exiting the cavity is measured as a function of wavelength. This OA-ICOS measurement is compared to one taken with no absorbing, sample in the cavity, along with a measurement of the ringdown time of the cavity (to determine the absolute loss due to the cavity mirrors), to generate an absorbance spectrum. This technique has advantages over traditional CRDS due to the simple experimental setup, and the fact the absolute alignment of the cavity relative to the laser beam is non-critical, which makes for a much more robust instrument. However, OA-ICOS technique has several disadvantages, including the measurement is taken at DC (i.e. there is no modulation on the light), so the effect of 1/f noise reduces the available sensitivity; the laser is coupled to the cavity during, a measurement, resulting in the laser's noise corrupting the measurement; and in order to achieve a dense mode structure, specialist astigmatic mirrors must be used.
The DC measurement used in OA-ICOS also does not allow the path length of the cavity to be measured (this is important for calculating an accurate absorption coefficient); such instruments need to measure the ringdown time throughout a measurement to calibrate the absorbance measurement from ICOS.